1. Field of the Invention
This invention relates to a superconducting six-axis accelerometer and more particularly to improvement in the accelerometer described in the inventor's prior publication, IEEE Transactions on Magnetics, MAG-21, 411 (march 1985), and prior U.S. Pat. No. 4,841,772 descriptions of which are incorporated by reference into the present disclosure.
2. Discussion of the Background
A six-axis accelerometer measures the linear and angular acceleration in all six degrees of freedom at the same point in space time. A superconducting six-axis accelerometer (SSA) operates at liquid helium temperatures and is able to take advantage of the corresponding low thermal noise and extremely high stability of materials and electric currents. The six-axis accelerometer uses the properties of quantized magnetic flux to magnetically levitate a single superconducting proof mass with extreme stability. The superconducting proof mass in the six-axis accelerometer is free to respond to acceleration, like the accelerometer in a superconducting gravity gradiometer.
In a three-axis superconducting gravity gradiometer, three pairs of coupled superconducting acceleration transducers are mounted on the six faces of a precision cube. Each acceleration transducer is a spring-mass type superconducting accelerometer, as shown in FIG. 1, in which an applied acceleration signal produces a displacement of proof masses m.sub.1 and m.sub.2. The proof mass displacements in the common and differential modes of motion are detected by a coupled superconducting inductance modulation circuit and superconducting quantum interference device (SQUID) amplifiers.
A simplified diagram illustrating the operation of a spring mass accelerometer is shown schematically in FIG. 2. Each accelerometer consists of the superconducting proof mass 20 confined to move along single axis and a spiral superconducting sensing coil 22 located near the surface of the proof mass 20. An acceleration will cause the displacement of the proof mass 20, which because of the Meissner effect will modulate the inductance of the coil 22 at frequencies down to dc. The sensing coil is connected to the input coil 24 of a SQUID amplifier 26 forming a closed superconducting loop. This loop contains a persistent current which couples the mechanical and electrical systems. Since the flux in this loop must remain constant, change in the inductance of the sensing coil results in a current change through the SQUID input coil 24. In this manner very small accelerations can be detected.
A superconducting accelerometer with a pair of spring masses is subject to errors caused by common accelerations that can seriously degrade the performance of the gradiometer because the ground has common accelerations which are several orders of magnitude larger than the extremely weak gravity gradient signals to be measured. As important error source of this kind comes from rotational motions which produce erroneous signals that are indistinguishable from gravity gradients. Although the errors along one of the three axes of a three-axis gradiometer caused by both torsional and tilting motions are minimized when that in-line axis is aligned with the vertical, such an orientation of the sensitive axis is not applicable to all three orthogonal axes simultaneously.
The six-axis accelerometer utilizes the same principles of the three axis gradiometer with two major differences:
1. Because the accelerometer has no common mode balance requirement only magnetic levitation (no mechanical spring) is used in the proof mass suspension. The proof mass resonant frequency can therefore be made low even in a simple design.
2. A single common proof mass has all the six degrees of freedom and can be conveniently shared by all the six component accelerometer circuits.
In the accelerometers in the gravity gradiometer, the proof mass position is confined by a mechanical spring and the accelerometer responds to acceleration in inverse proportion to the stiffness of this mechanical spring. On the other hand, the proof mass in the six-axis accelerometer is surrounded by quantized magnetic flux which confines the motion. Each degree of freedom responds to acceleration in inverse proportion to the stiffness of a "magnetic spring".
These magnetic springs are generated by the superconducting coils placed in close proximity to the proof mass. The proof mass excludes the magnetic flux from these coils due to the Meissner effect and any motion of the proof mass forces flux to redistribute itself within the superconducting circuitry. This transfer of energy produces a restoring force which opposes this change and creates electrical equivalent to a mechanical spring. Any displacement of the proof mass can be detected since the Meissner effect forces the inductance of sensing coils to change in proportion to the displacement of the proof mass.
Each degree of freedom is monitored by sensing coils connected together to form a superconducting bridge circuit. The sensing coils of each bridge are selected such that the output of each bridge is responsive to only one degree of freedom. The outputs of all of the bridge circuits are connected to a SQUID amplifier, converting the bridge circuit signals to an output voltage. Recovery of original displacement of the proof mass in each degree of freedom is accomplished through the use of lock-in amplifiers.
A simplified version of this detection technique is shown in FIG. 3 for an accelerometer with one degree of freedom. As proof mass 30 is displaced from equilibrium in the positive x-direction, the sensing inductances L2 and L4 decrease and the sensing inductances L1 and L3 increase. This change in sensing inductance unbalances bridge circuit 32 and an oscillating circuit proportional in amplitude to the misbalance appears across output coil 34. This current is then amplified and converted to an output voltage through the use of SQUID amplifier 36. Demodulation through the use of a lock-in amplifier recovers the original misbalance signal.